Relations between thymic factors and Prostaglandins (PG) were studied. We investigated on the effects of different incubation times with Thymosin Fraction 5 and Indomethacin on the release by spleen cells from normal or adult thymectomized mice. Prostaglandins were measured by radioimmunoassay. Thymosin induces an increase in PGE2 release on spleen cells obtained from thymectomized mice; the same effect was not observed on spleen cells obtained from normal mice.